dump bed ???

Ok....Is it a "Shaker" dump body?

 

Mine may be a little different since i pull dump trl, but sometimes I have to remove the cover at the rear of the slide cyl on pump and clean out road dirt because it limits travel. Don't pull cyl out though. The filter
I have is similar size of engine oil filter that spins on. But mine is on suction side between tank and pump, not on return

 

Have you tried bleeding the air out of the hoist?

 

Dump trucks don't typically have filters on the hydraulics. If you're sure the spool on the pump is moving freely check the airline that connects your raise and lower control to the air control on the pump for kinks or air leaks. I had the same problem before and found the plastic air line was kinked and not supplying full pressure to the pump.

 

I would bet it your air controller. You could just bypass it to see if it makes a difference

 

I've used some grain trucks that were slow to dump. When the bed is loaded and you need to lower it for some reason like material is bridging on one side, the oil has to return to tank with a much greater force on the cylinder (full bed). Pressure is force divided by area. The pressure trying to return to tank varies greatly (loaded versus unloaded) but area the area of the cylinder remains constant. Without getting into really expensive components the safest and cheapest solution is to design for the higher pressure returning to tank such that the bed doesn't drop so fast that the frame bends, hyd tank blows apart, breather is blown off, etc. So, the cheap solution is to place a restriction to flow (orifice) in the return path to tank after the hyd valve. Making this orifice larger will have a very big effect, if you do so you should make very small changes.

Apologies, i am too new to post web addresses it seems

Fixaddr.phtruck.com/download/Training/TrainingBasicHydraulics.pdf
Page 44 or 45 (pictorial schematic) and page 36 (bleeding air)

This is the hydraulic principle that helps mathmetically explain everything. If you knew what pressure the bed was at and measured velocity you could design for your new velocity by picking it, deriving the new pressure and back calculating the orifice area you need. Trial and error works, too! And so does a needle valve (variable orifice).
Fixaddr.engineeringtoolbox.com/bernouilli-equation-d_183.html

Good luck.

 

EARTHMOVER did you ever get the body to come down rite? if it was not the above mentioned i would agree with Gambi80 i have had a few dump trucks over the years and air in the piston will raise hell with it coming down!!!